1. Field of the Invention
This invention relates to a terminal interface device provided to additionally connect such a terminal unit having a central office line interface as an answering telephone set, a facsimile equipment, a cordless telephone set or a modem, to a main unit of a key telephone system without further providing a current supply circuit for additional terminal units and ringing signal generators.
2. Description of the Prior Art
There is a key telephone system in which a plurality of key telephones are connected through a main unit to a plurality of central office lines (CO lines). The main unit of such a key telephone system includes telephone interfaces connecting the key telephones to the CO lines, but does not include interfaces connecting between the system and a terminal unit equivalent to a standard telephone set having a central office line interface (CO line interface). For this reason, such a terminal unit as an answering telephone set, a facsimile equipment, a cordless telephone set or modem cannot be connected directly to the main unit of the key telephone system so as to commonly use the CO lines. For overcoming the above problem, such a method has been proposed in which in-service detectors are provided and terminal units with CO line interface are connected through the in-service detectors to each one of the CO lines leading to the main unit of the key telephone system, with the output of the in-service detectors being fed to the main unit. In this method, each terminal unit having the CO line interface must be connected to one of the CO lines in one-to-one correspondence. Accordingly, if a CO line connected to one of such terminal units is being used or serviced by the key telephone system, the terminal unit connected to the CO line cannot be used, that is, a calling from this particular unit cannot be made. When a ringing signal appears on a line other than the line connected to the terminal unit, this terminal unit cannot answer the ringing signal. In case that the terminal unit is an answering telephone, the answering telephone can answer the ringing CO line if the CO line is connected to the telephone but cannot answer other ringing CO lines which are not connected to this particular answering telephone. It is preferable that the answering telephone can exhibit its automatic answering function over all the CO lines connected to the key telephone system. However, according to the above method, the answering telephone cannot answer lines other than the particular one to which the telephone is connected and therefore this method is not preferable to provide an answering telephone as an option in the key telephone system.
As another method for connecting a terminal unit having a CO line interface to a key telephone system, it has been proposed to use a manually operated switch box to selectably connect the terminal unit to any one of plural CO lines. According to this method, switching of the switch box enables the terminal unit to function over all the CO lines leading to the key telephone system. However, in this method, the key telephone system and the terminal unit cannot use a CO line at the same time but must use it alternately. Thus, a "conference state" or a "branch", in which they are connected commonly to the same CO line, cannot be achieved. For this reason, in the event where the terminal unit is a facsimile equipment, key telephones in the key telephone system cannot make originating call for sending data from the facsimile equipment.